Contents

Hardware

Above is an annotated diagram of the circuit board. The dimensions of the board are approximately 121mm x 150mm — sized to fit comfortably underneath a standard-sized laptop keyboard. The image above is rotated versus the installation orientation; the port farm is meant to be on the right hand side of the laptop, not on the bottom. The overall height of the board is just under 14mm, with the height being set by the thickness of an Ethernet connector. The thickness on my Lenovo T520 base portion is just under 24mm, so once we stack a keyboard and plastics on this it’ll be just about the same.

Battery board

In order to give maximum power management flexibility, the battery interface functions are implemented on a daughtercard. I co-opt a cheap and common SATA-style connector to route power and control signals between the mainboard and the daughtercard. To prevent users from accidentally plugging a hard drive into the battery port, I inverted the gender on the mainboard of the battery-use SATA connector from the actual SATA-II connector. The current battery card is meant to work with the battery packs used by most RC enthusiasts — LiPo packs ranging from 2S1P to 4S1P (2-cell to 4-cell). RC packs are great because they are designed for super-fast charging. They are also cheap and easy to buy. For the board-side battery plug I decided to use the Molex connector found on classic disk drives, since they are cheap, common, and easy to assemble with simple tools. I couldn’t use a standard RC connector because the vast majority of them are designed for in-line use, and the few that have board mounts are too thick or too weird for use in this application.

The battery board can charge batteries at rates in excess of 4A. This means charging a 3-cell, 45Wh (4Ah) pack in about one hour. I’m estimating that a typical power consumption for a reasonable system configuration might be around 5-6W, so that’s 7-8 hours of runtime with a 1-hour charge time using that type of battery pack. Of course, since the whole laptop is user-configurable, typical power consumption is really hard to estimate — you could drop in a monster LCD and a power-hungry magnetic hard drive with loads of peripherals and the power consumption could be much higher. Of course, you can drop in a 100Wh battery pack if you wanted as well :)

Another cute feature of the battery board is that it can drive an analog panel meter. Xobs had suggested that it would be neat to embed a retro analog needle meter into the palmrest of the laptop to give a real-time display of power consumption. I thought it was a great idea, so I designed that in. Of course, the analog meter is driven by a DAC on the battery microcontroller, so it can be configured to perform a multitude of useful (or not so useful) read-outs, such as remaining runtime, battery voltage, temperature, the time (represented as an analog value), etc.

Firmware

Source

Mainboard

EVT-stage design files. As always, provided as-is, no warranty for fitness -- but particularly worthy of re-iteration since as an EVT design, it is in early stages and I fully expect to be finding and fixing bugs.

Battery board

EVT-stage design files. As always, provided as-is, no warranty for fitness -- but particularly worthy of re-iteration, since as an EVT design it is in early stages and I fully expect to be finding and fixing bugs.